A solid polymer membrane fuel cell, for instance, has a stack comprised by stacking a plurality of cells formed by sandwiching a solid polymer electrolyte membrane from both sides by an anode and a cathode, with hydrogen fed to the anode as fuel, and air fed to the cathode as an oxidant. Hydrogen ions generated at the anode by a catalytic reaction move through the solid polymer electrolyte membrane to the cathode, where they cause an electrochemical reaction with oxygen to generate electricity.
In fuel cells such as this kind of solid polymer membrane fuel cell, unreacted air discharged from the cathode (referred to as air off-gas) is normally discharged out of the system, in which case the hydrogen gas concentration in the air off-gas must be confirmed.
Systems have been developed in the past for confirming the concentration of hydrogen gas in air off-gas by, for example, installing a hydrogen detector in the discharge system of the cathode side of the fuel cell (see, for example, Japanese Examined Patent Application, Second Publication No. H06-52662).
A protective device is also known that shuts off the fuel supply when hydrogen at the fuel electrode side is detected to have leaked through the solid polymer electrolyte membrane to the oxygen electrode side by means of, for example, a gas sensor for detecting hydrogen gas provided in the discharge system of the oxygen electrode side of the fuel cell (see, for example, Japanese Unexamined Patent Application, First Publication No. H06-223850).
Use of a gas contact combustion-type gas detector is being considered in these hydrogen detectors. This gas contact combustion-type gas sensor comprises a detector element to which a catalyst is attached and a temperature compensating element to which a catalyst is not attached. Utilizing the heat of combustion when the detected gas (hydrogen in the case of a hydrogen detector) contacts the catalyst, it detects the gas concentration of the detected gas from the difference in electrical resistance between the detector element and the temperature compensating element.
Meanwhile, silicon in materials such as sealant used in a fuel cell can mix with the air off-gas, and when the hydrogen detector is exposed to this silicon during the catalytic reaction, the catalyst is poisoned. As a result, the hydrogen detector is degraded, and the detection accuracy drops. In addition, along with this silicon poisoning, degradation such as a drop in sensitivity due to sulfur poisoning and water absorption and the like may occur. Depending on the extent of degradation, replacement of the hydrogen detector may be required.
Accordingly, degradation diagnosis for hydrogen detectors is extremely important, and a method to easily diagnose whether a hydrogen detector is degraded or not is anxiously awaited.